DESCRIPTION: The mouse provides an excellent system for studying congenital cardiovascular abnormalities defects since mouse and human development are very similar and many mutants with heart and vascular anomolies have been identified (Harvey, 2002). Blood flow and the formation of the heart and vasculature coincide, and several studies suggest that mechanical stimulus, imparted by the shear stress between blood flow and the endothelial cell wall, acts as a signal for vascular remodeling (see Topper and Gimbrone, 1999 for review). Using confocal laser scanning microscopy, we have developed methods to measure blood flow velocities and calculate shear stress levels in mouse embryos (Jones et at 2004b). Here we will test the hypothesis that mechano-sensory signals are necessary for proper development of the yolk sac vasculature using three sets of in vivo, multispectral imaging experiments: First, we will study how the vascular plexus forms using celltype specific fluorescent protein expression. We will describe how endothelial cell morphologies change as vessels are assembled and flow begins. Second, we will quantify changes in shear stress during early yolk sac vessel formation and remodeling to identify forces that induces morphogenesis. Third, we will quantify hemodynamics in Myosin Light Chain 2a (MLC2a) mutant embryos (Huang et al 2003) to test whether disrupted flow and reduced shear stress can cause abnormal development of vascular cells in the yolk sac. MLC2a -/- embryos are ideal for studying the effects of shear stress because mutants have defects in vessel formation even though MLC2a is never expressed outside the heart. Through these Specific Aims, we will gather vital information about the dyanamic development of the yolk sac vasculature and will test how mechanical signals relate to changing vessel morphology. We have focused on a single mutant strain here, but the quantitative methods employed can be used to study any mutant with impaired vascular development or impaired cardiovascular function. [unreadable] [unreadable]